In transmitting a packet in a mobile communication system, a receiver should inform a transmitter of a presence or non-presence of success of a packet reception. In case that the packet reception is successful, ACK is transmitted to enable the transmitter to transmit a new packet. In case that packet reception fails, NACK is transmitted to enable the transmitter to retransmit the corresponding packet. This operation is called ARQ (automatic request).
The ARQ operation can be combined with channel coding scheme. In particular, the above-mentioned ARQ is proposed as HARQ (hybrid ARQ) which raises efficiency of whole system in a manner of lowering an error rate by combining a retransmitted packet with a previously transmitted packet. In order to raise throughput of system, the HARQ is requested to receive an ACK/NACK response faster than that of the conventional ARQ operation from a receiver. Hence, ACK/NACK is transmitted by physical channel signaling in HARQ.
Implementations of HARQ can be categorized into two types. A first type is chase combining (CC), in which retransmission is performed using the same code bits by the same modulation scheme and coding rate as that of a previously packet. A second type is incremental redundancy (IP), in which retransmission is performed in a manner of allowing a transmission by using a modulation scheme and coding rate different from those of the previously transmitted packet. In this case, the receiver can raise the throughput of system through coding diversity.
In a multi-carrier cellular module communication system, user equipments belonging to one or a plurality of cells perform uplink data packet transmission to a base station. Since a plurality of user equipments are able to transmit uplink data packets within a single subframe, a base station should be able to transmit ACK/NACK signals to a plurality of the user equipments within the single subframe. In particular, in the 3GPP LTE system, a base station transmits ACK/NACK signals to a plurality of user equipments via a physical HARQ indicator channel (hereinafter abbreviated PHICH), and more particularly, via a channel for transmitting downlink ACK/NACK information for uplink HARQ.
In case that a base station multiplexes a plurality of ACK/NACK signals transmitted to user equipments within a single subframe by CDMA in a partial time-frequency domain of a downlink transmission band of a multi-carrier system, the multiplexed signals are discriminated from ACK/NACK signals for other user equipments by orthogonal or pseudo-orthogonal code multiplied through the time-frequency domain. Moreover, in case of performing QPSK transmission, the discrimination can be achieved through two different orthogonal phase components. In particular, a plurality of ACK/NACK signals are transmitted by being multiplexed by CDMA through a plurality of PHICHs in the 3GPP LTE system. And, a unit of the transmission through the multiplexing by CDMA is called ‘PHICH group’.
Meanwhile, in case that a specific user equipment tries an initial access to a prescribed cell, the user equipment needs to acquire system information of the corresponding cell. Such basic information e.g. system bandwidth can be received via a physical broadcast channel (hereinafter abbreviated ‘PBCH’). Yet, in order to acquire detailed system information from the system information of the corresponding cell, the user equipment is requested to receive a physical downlink shared channel (hereinafter abbreviated ‘PDSCH’) that is the channel for transmitting general downlink data.
In this case, scheduling information of PDSCH is transmitted via PDCCH of each subframe. A user equipment in progress of an initial access receives PBCH and then receives PDCCH of a specific subframe. Hence, the user equipment is able to know scheduling information on PDSCH transmitting detailed system information through that subframe. In this case, in order to receive the PDCCH having the scheduling information about the PDSCH transmitting the detailed system information, a transmission location of the corresponding PDCCH should be known.
Since PDCCH is generally mapped to RE (Resource Element) except RE(s) for carrying PHICH and other control signals, it should be checked how the PHICH and other control signals are mapped to a resource region to receive the PDCCH.